Field of the Invention
The present invention relates to a sail for a wind-powered vehicle such as surfboard, sailing boat or the like.
Sails for vehicles of the kind mentioned above are widely known and there are several embodiments regarding their size and form. According to the wind situation although with a single vehicle a sail of an adapted size may be used in order to reach a high velocity at the one hand, and to diminish the danger of capsizing as far as possible. It is usual for sailing boats to reef the sailing area of the sail when required, i.e. to move the total sail downwards in relation to the mast, and to roll or fold the lower area of the sail. It is usual for sails of surfboards to use several differently sized sails for a single surfboard which are used according to the wind situation. A change of the sail area during the usage, however, is difficult and impractical.
A surfing sail is known from WO 91/11361 comprising a base sail which at its front edge is connected to a mast which in a usual manner can be connected to a bifurcate boom, and at its clew is connected to the other end of the bifurcate boom, and an additional sail serving for the reefing operation. The additional sail has a triangle shape and is cut narrow and high, and is pivotally connected at its head in the upper area of the base sail adjacent to the rear edge thereof, and in the lower area approximately at the level of the bifurcate boom is provided with a guiding web which is arranged and bent such that it is pivoted at a circular path when pivoting the additional sail with the circular path in the radius thereof corresponding with the distance of the guiding web from the fastening point where the additional sail is secured at the base sail. The guiding web is displaceable in the length thereof such that the additional sail may be pivoted inwards and outwards during the surfing.
With this surfing sail, the sail area is variable, however, there is the great disadvantage in practical operation that when varying the sail area this is accomplished for practical reasons only in the lower area shortly above the bifurcate boom. Thereby, at each variation of the sail area the center of area and thereby the pressure point of the sail is displaced upwards which results in a more difficult handling of the sail by the surfing person. A further disadvantage is that, close to the peak of the sail, a pivoting joint is required which in this area of the sail requires a corresponding rigidity which may be only attained by designing the mast in the peak thereof relatively rigid. Furthermore applications, in this case the pivoting joint in the peak area of the mast have a very negative influence on the weight distribution. The handling of the sail is better if the mast and sail are lighter in the top area thereof. Because of the lever forces which are caused by relatively large mast lengths modern sails in particular surfing sails and masts are made especially light in the top area. During the last years almost exclusively modern masts are used which comprise a so called flex dynamic, i.e. a strongly flexible mast top which yields when the wind pressure raises suddenly, and which thereby automatically decreases the sail area exposed to the wind for a while. This flex dynamic in modern sails is additionally supported by a sail top which is stiffened by sail laths and which comprises a twist dynamic which corresponding with the flex dynamic of the mast deforms the sail top area when the flexible mast top yields, and thereby decreases the sail area without the sailor or surfer having to operate it himself. The use of a mast with flex dynamic, and also the use of a sail with twist dynamic are not possible with the sails known from WO 91/11361 because of the construction.
Furthermore a surfing sail is known from GB 2 235 671 A wherein an adjustable sail part may be displaced upwards and downwards in parallel with the mast. The displacement of the adjustable sail part is attained by cable control with at least one of the cables guided over the top of the mast, and a deflection roller positioned there for pulling the adjustable sail part upwards, and in parallel with the mast, and securing it there by this cable control when the maximal area of the sail is required. For receiving the resulting forces with the deflection roller at the mast top the mast has to have a large stability and rigidity up to its top which results therein that with this known surf sail no mast with a flex dynamic in its upper area may be used. If somebody uses this known sail at a modern mast with a flex dynamic the disadvantage would result that the mast top with the full area of sails would be too soft, and with a small area of sails would be too hard considering the twist dynamics explained above. A change of the front edge length cannot be transferred to modern sails with a twist dynamic.